In the medical field, an X-ray apparatus refers to a device that transmits a predetermined amount of X-rays to a body part to be imaged, detects the transmitted X-rays with an X-ray sensor, and constructs an X-ray image based on detected electrical signals. The X-rays are attenuated and transmitted at different attenuation rates depending on the material on the traveling path, and when they reach the X-ray sensor, they are converted into electrical signals by photoelectric effect. The X-ray apparatus provides information about the inside of a subject as an X-ray image by using electrical signals with the cumulative amount of attenuation along the X-ray path being reflected in the image.
A computed tomographic (CT) image provides a three-dimensional X-ray image of the subject by reconstructing X-ray image data imaged at different angles while the X-ray generator that irradiates X-rays and the X-ray sensor that receives the X-rays are rotated around the subject.
A panoramic X-ray image is shown along an image layer as an arbitrary tomogram in the dental arch by superimposing a plurality of X-ray image data imaged by sections of the subject's dental arch through a so-called shift-and-add method, which is a tomography technique. The panoramic X-ray image is widely used particularly in the dental field.
The conventional panoramic X-ray image is problematic in that when a focusing area of the imaging apparatus, i.e., a reference image layer (predetermined image layer) determined by an imaging locus of the imaging apparatus is out of the actual dental arch locus of the subject, i.e., an interest area within the dental arch, the image of that part is not clear.
To solve the problem, there have been disclosed a technique for reconstructing a panoramic X-ray image of a plurality of image layers with a plurality of X-ray image data obtained through one imaging sequence (see Korean Patent No. 10-0917679), and a technique for providing a panoramic X-ray image that approximates the dental arch locus over the entire section of the panoramic X-ray image by choosing and combining the sharpest panoramic X-ray image that is closest to the dental arch locus through comparing panoramic X-ray images of a plurality of image layers by sections (see Korean Patent No. 10-1094180). There have been further disclosed a technique where after sizes of the images reconstructed along the image layers or sizes of the images used for reconstruction are scaled to be equal to the size of a reference image, all or a part of an image that clearly shows a predetermined interest area among the scaled images is selected, and the scaled images are divided into a plurality of blocks and the sharp images are selected from the divided block images, and a panoramic X-ray image is provided by using the selected images (see Korean Patent No. 10-1389841), and a technique where in the state where image data of a plurality of image layers is stored, a reference image layer is determined from a plurality of image layers, and a block corresponding to at least one block specified in the reference image layer is found in another image layer, after the clearest image is selected by comparing image data of the corresponding blocks, when the selected block is not the block specified in the reference image layer, the reference image layer is reconstructed and displayed by replacing the specified block of the reference image layer with the selected block of another image layer (see Korean Patent No. 10-1664166).
The panoramic X-ray image has the depth resolution, that is, the spatial resolution in the depth direction according to the X-ray irradiation direction, due to the characteristics of tomography technology. Here, the depth resolution of the panoramic X-ray image is inversely proportional to the depth of the image layer according to the X-ray irradiation direction, i.e., the thickness of the image layer. However, the conventional panoramic X-ray image is insufficient in the aspect of a depth resolution compared with computed tomography. For example, at the present level of panoramic X-ray image, it is difficult to distinguish the depth difference between the inner dental root and the outer dental root from a single image in one molar. Accordingly, some diseases such as periodontitis may not be found in panoramic X-ray images depending on where they occur.
A cephalometric X-ray image is a two-dimensional X-ray image of a head portion, which is divided into two methods: one-shot method for reconstructing a two-dimensional X-ray image of the imaging area with one directional X-ray image data transmitted through the entire imaging area; and scan method for reconstructing reconstructs a two-dimensional X-ray image of the imaging area with a plurality of X-ray image data obtained by scanning an X-ray passing through a part of the imaging area in the width direction. The cephalometric X-ray image is mainly used in the dental or ENT fields and is classified into LAT (lateral), AP (anteroposterior), PA (posteroanterior), SMV (submento vertex), and W/V (water's view) depending on the imaging direction.
However, since the cephalometric X-ray image is a two-dimensional X-ray image without depth resolution, that is, spatial resolution in the depth direction according to the X-ray irradiation direction, it is impossible to distinguish the desired, and as a result, to identify the section at a specific depth in a cephalometric X-ray image, computed tomography of the entire head must be performed, which causes unnecessary overexposure as well as economic burden on the subjects and hospitals.